1. Field of the Invention
The invention relates to a method and device for determining the redox state of the anode of a high-temperature fuel cell, which is coated with or made from catalyst material, or of the reaction surface of a reformer, and further to a piezoelectric sensor device for determining the redox state of an oxidizable/reducible coating.
2. The Prior Art
High-temperature fuel cells, such as e.g. the solid oxide fuel cell or the molten carbonate fuel cell, must be supplied with gases at the electrodes during operation, i.e. combustion gas (H2, CO or CH4) at the anode and oxidizing gas (O2 or air) at the cathode. The gas spaces must be sealed against each other. Insufficient sealing results in a reduction of the cell voltage and usually leads to degradation and failure of the fuel cell.
In addition, if oxygen enters an H2-enriched anode space at temperatures below 600° C., a highly explosive mixture will result.
Nickel (Ni) or nickel-cermet may be used as catalyst material at the anode, but this will form nickel oxide (NiO) in contact with an atmosphere containing oxygen, and its catalytic activity will deteriorate. For this reason starting the operation of a high-temperature fuel cell with Ni or Ni-cermet as anode material requires a so-called reduction phase during which the anode space is initially flushed with nitrogen and is then subjected to the combustion gas (e.g. hydrogen), which acts as reducing agent and whose concentration is increased in a stepwise manner. During this reduction phase NiO is reduced to metallic Ni. Conversely, oxidation of the anode material cannot be avoided on certain occasions, e.g. during maintenance activities. Reduction and oxidation of the anode is referred to as the “redox cycle”.
It is known practice to determine such redox cycles, or the redox state, of Ni/YsZ-cermet anodes (YsZ=yttrium doped zirkonium) via their polarisation state, for instance by impedance spectroscopy. This method is used primarily in the development of fuel cells, but reliable inferences regarding the catalytic activity of the anode are only possible in single cell experiments. The method is not suitable for continuous monitoring of high-temperature fuel cells during operation.
From “Kinetics of Oxidation and Reduction of Ni/YsZ Cermets”, 5th European Solid Oxide Fuel Cell Forum, Vol. 1 (2002), pp. 467-474, authored by Daniel Fouquet, Axel C. Müller, André Weber, and Ellen Ivers-Tiffée, there have become known findings concerning the kinetics of the reduction and oxidation of NiO/Ni powder and NiO/Ni-YsZ cermets, obtained with the use of TGA measurements (Thermo Gravimetric Analysis). Measurements of this kind cannot be used for the continuous monitoring of high-temperature fuel cells, however.
Reformers, e.g. vapour reformers for the generation of hydrogen-rich combustion gas for fuel cells, have reaction surfaces coated with catalyst materials, at which a combustion gas containing H2 and CO is obtained, for instance from the primary media natural gas and water vapour. Reformers working with methanol as a primary medium are also used to generate a hydrogen-rich combustion gas according to the equationCH3OH+H2O+heat→3H2+CO2 Different redox layers, e.g. Ni/NiO or nickel-cermet, are used as catalyst materials. Efficiency and operational safety of the reformer depend on the state of the reaction surface.
From U.S. Pat. No. 6,455,181 B1 a fuel cell assembly is known which has a sensor with a membrane electrode, whose one side is connected with the gas feed of the fuel cell and whose other side is connected with the exhaust of the fuel cell, the two sides having different coatings. The different composition of the gas on the two sides of the membrane, for instance a difference in hydrogen ion concentration, is converted into an electric signal, which can be used to control the gas flow of the fuel cell. Thus differences in the gas concentration at the inlet and the outlet of the fuel cell are measured, but the device cannot be used for determination of the redox state of the anode of a high-temperature fuel cell or the redox state of the reaction surface of a reformer.
It is the object of the present invention to propose a method and device as well as a sensor arrangement for the monitoring of the redox state of the anode of a high-temperature fuel cell or of the catalytic reaction surface of a reformer, which may be used during normal operation and which should furthermore ensure optimised and safe operation of the fuel cell or fuel cell assembly including the reformer, by controlling or adjusting at least one operational parameter.